The present invention relates generally to cellular-type communication systems. The present invention has application, for example, as part of a radio receiver using digital signal processing circuitry to perform pilot strength searches.
Large-user radio communication systems have been developed into a complex inter-networked web of systems deployed to provide coverage internationally. Examples of such systems include cellular radio communication systems and other wireless systems such as wireless LANs. A cellular radio communication system includes multiple communication cells arranged adjacent one another to cover a larger regional area. Each cell limits the number of possible simultaneous communications to the number of channels provided in the cell. The size of the cell is defined through receivers and transmitters (a.k.a., xe2x80x9ctransceiversxe2x80x9d) located within base stations that provide the communication channels through which the mobile radios communicate. A mobile radio communicates in a cellular system by communicating first with a number of close-proximity base stations before selecting the best or nearest base station with which to establish a radio communication link.
In providing a clear channel between the mobile radio and the selected base station, the prevention of interference from and to other radio communication links is an important concern. Generally, this concern is addressed through the use of an accurate method for determining the nearest base station and by controlling the transmission power levels used in maintaining the communication. If the nearest base station is not accurately selected or changes without a timely update, the communication can overlap and interfere with other communications in the system.
In the context of a specific system such as a cellular communication system, the mobile stations select the nearest base station by monitoring a control channel transmitted from each base station for its signal strength and selecting the nearest base station in response to comparing these channel reception levels for the best signal quality. The transmission power is controlled at minimum levels by using algorithms at the base station and/or the mobile radio and, in some systems, also by passing control information between the base station and the mobile radio during the communication. Many of these systems use direct-sequence, spread-spectrum (DSSS) code-division-multiple-access (CDMA) communication. In this type of system, the same frequency is commonly used by a plurality of users by breaking apart the communication and transmitting using different codes. At any given frequency, the signals of other users interfere with the measurements for the signal quality of the transmitting base stations. In an attempt to account for this interference, measurement methods determine the signal quality by computing the ratio of the signal reception level and the interference level, which is known as xe2x80x9cSIRxe2x80x9d or Signal to Interference Ratio, as described for example, by T. Dohi, et al: xe2x80x9cPerformance of SIR Based Power Control in the Presence of Non-uniform Traffic Distribution,xe2x80x9d 1995 Fourth IEEE International Conference on Universal Personal Communications Record, pp. 334-338, November 1995. In addition to the SIR, the signal quality determination is also dependent on the estimates of the signal reception level which, in turn, are dependent on the signal propagation and the accuracy in which these estimates are communicated, a problem known as xe2x80x9cmulti-path Rayleigh fading.xe2x80x9d Multi-path Rayleigh fading (xe2x80x9cfadingxe2x80x9d) is caused by reflections of the signals being received from the base stations through the wireless communication medium.
Communication of measured radio frequency (RF) signal strength is also subject to such fading. In CDMA-based cellular communication systems, the mobile station""s selection of the nearest base station often consists of a search for the optimal CDMA pilot strength of beacon signals transmitted from each base station to determine whether a handoff from the current base station to another base station should occur. Within each mobile station, the transmitted beacon signal is demodulated in a spread spectrum receiver using a correlation process in which the received signal is correlated with a replica of the signal used in the transmitter to spread the signal through the bandwidth. For spreading direct-sequence spread spectrum (xe2x80x9cDSSSxe2x80x9d) signals, the signals are typically spread using a pseudo-random noise binary sequence or (xe2x80x9cPNxe2x80x9d code). The receiver despreads the DSSS signals by proper synchronizing the spreading waveform with a replica of this same PN binary sequence.
The pilot beacons of each base station use the same PN code and are differentiated by unique offsets to the sequence. The estimated strength of a PN-code offset associated with a base station (i.e., pilot strength) is used by the mobile in the idle mode to decide which base station to perform a handoff to. The estimation process is referred to as xe2x80x9csearching,xe2x80x9d which involves correlating the input signal with a locally generated PN-sequence with the corresponding offset. The handoff decisions taken in the idle mode (Idle Handoff) of the mobile station operation are purely mobile-controlled, i.e., without any base station interaction.
The cellular base stations transmit a (General/Extended) Neighbor List Message that contains the PN-code offsets (often called pilot PN) of all nearby base stations such that the mobile can concentrate on searching them. Each neighbor pilot is also associated with a priority and a window size. Since not all neighboring base stations are as likely to have a powerful enough signal in the coverage area of the base station transmitting the Neighbor List Message, the priority indicates how important it is to search that pilot. Since all base stations are not equidistant from the present base station, the window indicates the range of PN-code offsets to search for detecting the corresponding base station; in general, the longer the window specified, the farther away the base station is from the signal-receiving mobile station. While no guidelines specify how the mobile should use these priority and window data to perform the searches, a typical implementation involves searching the entire window (corresponding to a pilot PN) in its entirety before moving on to another pilot, irrespective of the window length and priority of the pilot.
A problem with implementing this searching approach involves the potentially-conflicting concurrent duties carried out by the mobile station. For example, the mobile station""s Idle Mode includes a slotted-mode operation in which the mobile station only monitors the strengths of the base station for a short period of time (e.g., about 80 ms) in relatively long time intervals (e.g., 1.28 s). Hence, even with a relatively fast searcher it is not possible to search all the PN-codes necessary to cover all the windows of the neighbors within a typical channel-coherence time. Moreover, the time required to search a large window may be too large relative to the wake-up period (e.g., 80 ms).
Accordingly, there is a need for an improved approach to pilot-strength searching and related signal processing in such communication systems.
According to various aspects of the present invention, embodiments thereof are exemplified in the form of methods and arrangements involving a CDMA receiver implemented to search for the pilot signals in a more efficient and accurate manner.
According to the present invention, an example application is directed to a CDMA communication system involving searching of a set of pilot signals, including a first pilot signal having a search window length that is greater than a threshold number of chips and including a second pilot signal having a search window length that is not greater than the threshold number of chips. The search window for the first pilot signal is decimated into a plurality of sub-windows, and then the second pilot signal and one of the sub-windows is searched using a common prioritization criteria.
Another example application of the present invention is directed to methods and arrangements for conducting such searching of the pilot signals at the mobile station. A more particular aspect of the present invention is directed to a specific CDMA cellular application. In this application, a communications system includes a mobile station communicating with a plurality of base stations. The pilot search results are processed in the mobile station""s receiver to provide samples of the search results for each PN code. These samples are processed to determine which of the pilot signals represents the strongest.
In another specific implementation, base stations are located to cover a designated geography by communicating with mobile stations in respective geographic cells. Each base station generates a pilot signal, so that the base stations collective generate a set of pilot signals, including a first pilot signal having a search window length that is greater than a threshold number of chips and a second pilot signal having a search window length that is not greater than a threshold number of chips. Each mobile station includes a signal processor circuit that is configured and arranged to search the set of pilot signals, to decimate the search window for the first pilot signal into a plurality of sub-windows, and to search the second pilot signal and one of the sub-windows based on a prioritization criteria that is common to each of the second pilot signal and one of the sub-windows.
Another example CDMA-based cellular application is directed to a method of combining the priority and the window size information in order to sequence the searches in the idle mode such that pilots with higher apriori probability of being a candidate for idle handoff are searched more frequently, and all pilots, regardless of window size are given a fair chance at getting searched, and hence increases the probability of Idle Handoffs occuring promptly. This in turn lowers the probability of the Active Set signal strength from falling below any particular levelxe2x80x94thus increasing the reliability of paging message reception.
The above summary is not intended to provide an overview of all aspects of the present invention. Other aspects of the present invention are exemplified and described in connection with the detailed description.